Conventionally, an external control type fan coupling device has been disclosed, which consists of a combination of an electromagnet and a fluid coupling, a combination of an electromagnetic clutch and a magnet type coupling, or a combination of an electric actuator and a fluid coupling, and a method for controlling a rotation speed of a fan has been disclosed, which uses a control system such as PID control (feedback control system based on proportion, integration and differential) and adaptive control based on sensor output signals such as an accelerator opening degree signal, an internal combustion engine coolant temperature signal, an internal combustion engine rotation speed signal, etc., in regard to the external control type fan coupling device. (For example, refer to Patent Documents 1 through 7).    Patent Document 1: Japanese Published Patent Application No. 2003-239741 A    Patent Document 2: Japanese Published Patent Application No. 2004-340373 A    Patent Document 3: Japanese Published Patent Application No. 2002-195303 A    Patent Document 4: Japanese Published Patent Application No. 2003-156072 A    Patent Document 5: Japanese Published Patent Application No. 2006-162062 A    Patent Document 6: Japanese Published Patent Application No. 2006-162063 A    Patent Document 7: Japanese Published Patent Application No. 2006-112466 A